Forgetting to drink sounds almost as silly as forgetting to breathe, yet there's a dehydration epidemic, according to a new Harvard study. Researchers found that over half of 4,000 kids studied weren't drinking enough, with 25 percent saying they didn't drink any water during the day. And this isn't just a kid problem: A separate study found that adults may be doing an even worse job of hydrating. (This is Your Brain on Dehydration.) Up to 75 percent of us could be chronically dehydrated!

Being a little low on water won't kill you, says Corrine Dobbas, M.D., R.D. But itcan decrease muscle strength and aerobic and anaerobic ability, as well as cause poor mental performance, headaches, and make you feel sluggish, she says. So how do you know if you're drinking enough H2O? Your urine should be pale yellow or very clear, says Dobbas. But there are several other less-obvious signs your water tank needs a refuel.

You're Hungry

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When your body wants a drink, it's not picky about where that water comes from and will happily accept food sources as well as glass of plain water. That's why many people assume they're hungry when they start to feel weak and tired, Dobbas says. But it's harder to get hydrated through food (not to mention more caloric!), which is why she advises drinking a cup of water before eating to see if that takes care of your "hunger." And if your mouth is craving something more flavorful, try flavored waters or these 8 Infused Water Recipes.

Your Breath Reeks

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One of the first things to get cut when you're dehydrated is your saliva production. Less spit means more bacteria in your mouth and more bacteria means stinky breath, according to research published in the Orthodontic Journal. In fact, the study authors write that if you go see your dentist about chronic halitosis, usually the first thing they suggest is drinking more water—that often takes care of the problem.

You're Grouchy

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A bad mood may start with your water levels, according to a recent study published in the Journal of Nutrition. Scientists found that young women who were just one percent dehydrated reported feeling more anger, depression, annoyance, and frustration than women who drank enough water during a lab test.

You're a Little Fuzzy

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That afternoon brain drain may be your body crying for water, according to a study in the British Journal of Nutrition. Researchers found that people who were mildly dehydrated during the experiment performed worse on cognitive tasks and reported feelings of wanting to give up and an inability to make decisions.

Your Head Is Pounding

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That same study that found that dehydration increased moodiness in women also found an increase in headaches in the dried-out ladies. The researchers added that dropping water levels could decrease the amount of fluid surrounding the brain in the skull, giving it less padding and protection against even mild bumps and movement.